Hmmm.... so plains become forests and forests become marshes.
@Fulmen ?
Some provinces could probably do with becoming forests and marshes, yeah. Finland in her modern, post-1947 borders is the 6th most swampy country in the world. Particularly Northern Finland is swampy, with 65% of the country's swamps being there. If I'm not mistaken, this "swamp belt" extends into Northern Sweden as well.
Here's a map of Northern Finland in modern borders with swamps and how much they make up of the land area. The data is from 1960, but shouldn't be much different to the 1930s-40s or indeed to today.
What I particularly had in mind though, was a more general assessment of Finland, particularly the approaches into the country where most of the fighting took place historically.
Here's the Karelian Isthmus again, with fortification lines from the Continuation War highlighted. Quick note here: barring the Salpa Line, most of the fortifications were badly under construction by mid-1944. The VKT Line, where the Russians were stopped in June-July 1944, for example existed in practice only on the map, with only very limited fortification work having been done on it by then.
As you can see, the Isthmus is a bottleneck, and the large amount of rivers and lakes limits the movement of attackers. The Vuoksi river for example forms a natural barrier where the Finns held during the entire Winter War and in the last months of the Continuation War, and the VKT Line, over half of it on the Vuoksi, while not fortified, was geographically the most defensive out of the three lines on the Isthmus.
Here's most of Ladoga Karelia. As you can see it's not a bottleneck wedged between the sea and a giant lake in the same way the Isthmus is, but it is one in other ways: limited infrastructure. The comparatively few roads in the region tied the Russians to them, limiting mobility, allowing for much smaller forces to contain, even pocket them (e.g. the Lemetti
motti, visible on
this map from my earlier post). The Finns, who could ski and were generally far more adept at forest warfare than the Red Army, were much more mobile and less tied to the roads.
EDIT: The lakes and rivers in Ladoga Karelia of course also created local bottlenecks.
North of that it only gets worse, with a whole lot of forest and few roads. In the Winter War the Russians were crazy enough to attack from the width of the entire eastern border. Turns out sending entire divisions, mechanised divisions at that, in the middle of nowhere, tied to a single dirt road, isn't exactly the best of ideas. Battles like Suomussalmi and the Raate Road were the result.
Had the Russians gotten past the frontier regions and into the inland of the country, the terrain and infrastructure wouldn't have improved for them much. Take a look at this map, it outlines the main roads, railroads and the bigger lakes of Finland and East Karelia.
Not the kind of region where one can execute large armoured manoeuvres like on the steppes of the Ukraine.
Come think of it, maybe they should add another terrain type to represent areas with a ton of lakes too small to represent on the map, similar to what they did by adding the fjords and archipelagos sea zone type, because making them marshes isn't really accurate. I'm talking about areas that could hold a lot of tanks, aren't quite marshes as a whole, but had a lot of lakes, restricting the manoeuvrability of forces, vehicles in particular. Areas like the Karelian Isthmus, and honestly most of Finland.